This invention relates to a microelectromechanical systems (MEMS) thermal switch and its method of manufacture. More particularly, this invention relates to a MEMS thermal switch having a tether slideably engaged with a hot, expanding beam and rigidly engaged with a cool, passive beam.
Microelectromechanical systems (MEMS) are very small moveable structures made on a substrate using lithographic processing techniques, such as those used to manufacture semiconductor devices. MEMS devices may be moveable actuators, sensors, valves, pistons, or switches, for example, with characteristic dimensions of a few microns to hundreds of microns. A moveable MEMS switch, for example, may be used to connect one or more electrical input terminals to one or more electrical output terminals, all microfabricated on a substrate. The actuation means for the moveable switch may be thermal, piezoelectric, electrostatic, or magnetic, for example.
A thermal electrical switch may be formed, for example, by disposing an expanding beam adjacent to a passive cantilever, and causing the expanding beam to expand, thereby deflecting the passive cantilever. In one known embodiment, the expanding beam is a conductive circuit through which a current is driven to heat the conductive circuit. The conductive circuit may be tethered to the passive cantilever, also called herein a flexor beam, by a dielectric tether, such that the current does not flow to the passive cantilever from the conductive circuit. The conductive circuit may heat from Joule heating and expand relative to the passive cantilever, thus bending the passive cantilever to which it is tethered. If the passive cantilever is coupled to an electrical input terminal carrying an electrical signal, energizing the conductive circuit may deflect the passive cantilever to a position in which it is in contact with another electrical output terminal, thereby connecting an input terminal to an output terminal. The conductive circuit and passive cantilever may therefore constitute an electrical switch
The dielectric tether may therefore be coupled to both the conductive circuit as well as the passive cantilever. The expansion of the conductive circuit generates a force in the longitudinal direction, which is converted into a lateral, bending force by the rigid attachment of the dielectric tether. The dielectric tether of this thermal switch therefore applies a force in a lateral direction against the passive cantilever, in order to cause the passive cantilever to bend laterally in the desired direction However, because the conductive circuit expands longitudinally, the expanding beam also exerts a shear force on the dielectric tether, in a direction orthogonal to the desired bending direction Accordingly, the dielectric tether may be required to accommodate a substantial amount of shear force while converting the shear force to the desired lateral force. Excessively large shear forces may lead to cracking and delamination of the dielectric tether.
Therefore, in order to avoid large stresses, the dielectric tether may be made from a relatively compliant material. The compliance of the material may reduce the efficiency of the device, however, because some amount of the force exerted goes into the deformation of the compliant material, rather than to the deflection of the passive cantilever in the desired direction. Accordingly, using compliant materials reduces the mechanical and thermal efficiency of the device, by requiring higher temperatures to produce the desired amount of deflection in the passive cantilever.